- As the global demand for natural resources increases, more land will be intensively managed for the production of commodities such as timber, with potential consequences to biodiversity, ecological functioning and ecosystem services provided to society. Although there is strong consensus that intensive land management practices can negatively affect biodiversity, less well understood are the ways that management and biodiversity-associated processes interact. In managed forest systems, wild ungulates have caused shifts in forest succession by selectively browsing regenerating trees and have promoted exotic species proliferation by negatively affecting native biodiversity. However, few studies have addressed the ways that management intensification alters the effects of wild ungulate herbivores on plant community assembly, plantation development and native species conservation.
To address this knowledge gap, I used data from a six-year experimental dataset to test hypotheses pertaining to the interactive effects of forest management intensity and herbivory by both black-tailed deer (Odocoileous hemionus colombianus) and Roosevelt elk (Cervus elaphus roosevelti) in the Oregon Coast Range, USA. Specifically, the experiment manipulated early forest plantation management intensity via herbicides and deer and elk access to vegetation via exclosures. The six-year dataset included annual measurements of plant species and crop tree responses to herbivory and herbicide and the documentation of ungulate foraging via camera traps. In Chapter 1, I present the theory, management history and initial findings from my Master’s thesis that helped formulate the hypotheses for the dissertation, which are specifically addressed in Chapters 2-4.
For Chapter 2, I hypothesized that by simplifying community composition at local scales, intensive forest management practices cause plant communities to be more susceptible to herbivore-mediated changes across time and space. This hypothesis relates to the initial findings of my Master’s thesis, which provided evidence that the initial effects of herbicide on plant communities were mediated by the effect of ungulate herbivory (Stokely 2014). To test this hypothesis, I used six years of plant community data to test the interactive effects of herbivory and herbicide treatments on the changes in plant community composition, functional group associations and diversity quantified at multiple scales of species assembly. I found evidence that selective foraging by deer and elk slowed developmental changes in community composition, resulting in divergent plant communities, but the strength and direction of the effect depended on management intensity. The effect of herbivory was strongest in a commonly used management regime that initially suppressed plant diversity and native broadleaf assemblages, thereby promoting the abundance of exotic herbaceous species. Selective foraging suppressed the succession of broadleaf species and further promoted the homogeneity of herbaceous assemblages among stands, resulting in lower beta and gamma diversity.
For Chapter 3, I hypothesized that wild ungulates provide an ecosystem service in forest plantations when selective browsing of non-crop vegetation reduces competition and increases crop-tree growth, but the direction and magnitude of the service should vary by management intensity at stand and landscape scales. This hypothesis relates to initial findings from an early manuscript of the study (Stokely et al. 2018), which revealed that deer and elk improved crop-tree survival at Lighter management intensities. To test the hypothesis, I used a five-year dataset of Douglas-fir (Pseudotsuga menziesii) volume, stand-scale broadleaf cover (i.e., angiosperm shrubs and trees) and landscape-scale broadleaf habitat measurements as well as deer and elk camera trap detections to assess the prevalence of foraging ungulates in plantations. I found evidence that deer and elk provided an ecosystem service in intensively managed plantations by further suppressing the development of broadleaf species, thereby promoting crop-tree growth. Alternatively, deer and elk negatively affected crop-tree growth in less intensively managed stands where crop-trees were unable to outgrow competition and browsing. The ecosystem service increased with the prevalence of deer and elk in intensively managed stands, which was positively associated with both stand and landscape-scale abundance of broadleaf habitat. This finding indicates that surrounding broadleaf habitat is an important factor in providing a source of herbivores to plantations, which serve to further suppress stand-scale broadleaf regeneration and provide the ecosystem service we observed.
For Chapter 4, I hypothesized that by suppressing native plant species, wild ungulates and management intensification synergistically promote the proliferation of exotic species. This hypothesis relates to initial findings from the Master’s thesis, which revealed that herbicides and herbivory promoted exotics at certain management intensities (Stokely 2014). To test this hypothesis I used a six-year dataset of native and exotic species responses to management intensity and herbivore exclusion. I found evidence that by initially reducing the diversity and abundance of native species, management intensification caused native species to be more susceptible to herbivory, which further promoted the proliferation of exotics.
The findings of Chapters 1-3 indicate that by initially suppressing plant diversity, intensive management practices (i.e., herbicide treatment) altered early successional plant community assembly, promoting the abundance of exotic herbaceous species. The altered composition of heavily managed plantations created susceptibility to the effects of herbivory, which further suppressed native broadleaf succession and lengthened the duration of the herbaceous stage. By suppressing broadleaf species, deer and elk provided an ecosystem service to plantations, increasing both the survival and volume growth of Douglas-fir trees. Despite apparent browse damage in those stands, the lack of competition allowed crop trees to accrue biomass and eventually release above regenerating competitors and browsing herbivores. When broadleaf species were retained at lighter treatments, the combined effects of browsing and competition inhibited the release of crop trees above competitors and browsing deer and elk. The positive ecosystem service provided by deer and elk in intensively managed plantations had negative consequences for native early successional species, further promoting exotic species proliferation.
The findings presented in this dissertation further support for the need for ecological research to account for multiple interacting disturbances and ecological processes. Additionally, the scale at which plant community responses are quantified and perceived effects of management are considered may provide alternative interpretations of the effects of interacting disturbances. In Chapter 5, I present an overview and synthesis of these findings and discuss the mechanisms responsible for our observations as well as management implications and recommendations.